1. Field of the Invention
The present invention relates to a method and apparatus for dehydrating sludge including the step of charging a preliminary dehydrator with a wet sludge and performing preliminary dehydration of the sludge and the step of performing compression dehydration of the predehydrated sludge using an endless filter fabric
2. Description of the Prior Art
A conventional method is known for dehydrating industrial waste or sludge from a sewage treatment so as to allow easy disposal and facilitate combustion. According to this method, as shown in FIG. 1, a wet sludge 3 is uniformly supplied onto a filter fabric 1 to perform gravity dehydration. Thereafter, the obtained sludge 3 is sandwiched between the filter fabric 1 and another filter fabric 2 to perform compression dehydration. When the sludge 3 is compression-dehydrated by the two filter fabrics 1 and 2, water content 4 in the sludge 3 is removed only in a vertical direction through the filter fabrics 1 and 2 and is not removed in a front-to-rear direction or right-to-left direction. As a result, removal of the filtered water is delayed and the water content in the sludge cannot be decreased beyond a certain degree.
Since the water in the sludge is removed only in the vertical direction through the filter fabrics, when clogging of the filter fabrics 1 and 2 occurs, the water separated from the sludge 3 cannot be easily removed through the filter fabrics 1 and 2.
In addition, when high pressure is applied to the sludge before its water content is decreased to a satisfactory degree, the sludge 3 rolls and leaks from the filter fabrics 1 and 2, thus disabling dehydration.
A conventional dehydration apparatus as shown in FIG. 1 has the following problems:
The first problem is a low dehydration efficiency since only one type of filter fabric is used for both gravity dehydration and compression dehydration of the sludge. In general, a filter fabric having a large filtering area and a large mesh is suitable for gravity dehydration, while a filter fabric having a small mesh is suitable for compression dehydration. In view of this, if only a single type of filter fabric is used for gravity and compression dehydration, the dehydration efficiency is degraded.
The second problem is a low dehydration treatment capacity due to a low gravity dehydration efficiency. As described above, when sludge having a high water content and a high flowability is sandwiched between two filter fabrics and high pressure is applied, the sludge may leak from the filter fabrics If the gravity dehydration efficiency is low for this reason, the compression dehydration efficiency will be adversely affected. Thus, the overall dehydration treatment capacity is degraded.
The third problem is a low decreasing rate in the water content of the sludge. Since a conventional sludge dehydration apparatus has filter fabrics simply wound around rollers, a maximum compression pressure of only about 2 kg/cm.sup.2 can be applied. This pressure does not result in a satisfactory decrease in the water content of the sludge.